In the prior art, the method of construction of face-mounting transducer elements is to carry the deformation of the acoustic face back through the transducer elements to a stack of pressure-release paper mounted against a load-carrying plate. A pressure-release material may be briefly defined as one whose acoustic impedance, Z=.rho.c, is less than that of water, where .rho.=the density and c=the propagation velocity of the material. The stack might consist of as many as 96 sheets of onionskin paper. Operation depth-wise is limited, since paper is not a linear elastic material, and acoustic isolation of the transducer elements from the load-carrying plate is not complete, since the acoustic impedance of paper is not as low as it should be, and therefore the operating depth of the transducer assembly is impaired.
It is desirable to have the acoustic impedance at the back face of the heads of the transducer elements as low as possible to minimize acoustic coupling into the backing structure. This low impedance may be achieved at any frequency by designing the pressure-release material, array support structure, and the backing plate to be a resonant, acoustic transmission-line matching section. However, to maximize the transducer's bandwidth, the characteristic impedance of the pressure-release material should be as low as possible and should have small variation with changes in pressure. The pressure-release paper of the prior art is not elastically linear and therefore detunes the acoustic elements. This is so because, since the paper is elastically nonlinear, then the equivalent mechanical reactance presented to the transducer varies as a function of the amount of compression of the nonlinear material. This is analogous to a varying reactance across a resonant circuit, which detunes it.
In this invention, the pressure-release paper is replaced by a material which may be the proprietary produce called Min-K 2000, which consists essentially of silica particles in a phenolic binder, manufactured by the Johns-Manville Company, Research and Engineering Center, P. O. Box 159, Manville, N.J. 08835. In more detail, Min-K 2000 is a combination of amorphous silica and crystalline rutile, held together by asbestos fibers and phenolic resin.
A process for making a similar material is fully described in the patent having the U.S. Pat. No. 3,542,723, dated Nov. 24, 1970, entitled "Method of Molding Aggregate Pressure Release Material for Deep Submergence," and assigned to the same assignee as the subject application. Essentially, therein described a process for making a linearly elastic, pressure-release, material which retains substantially linear acoustic properties after going through a process comprising the steps of: grinding silica into finely divided particles; mixing the silica particles with a phenolic binder; forming the mixture of silica and binder into a desired structural element; and subjecting the structural element to a prestress which exceeds the operating stress at which the material is subsequently to be used.
The silica particles may be in either morphous form, such as quartz particles, or in amorphous form such as glass particles.
The material just described and the MIN-K material, either of which may be used for the purposes of this invention, will henceforth be termed by the generic term "siliceous material" or "siliceous particles."
Comparative measurements have been made on the pressure-release paper and the siliceous material; the behavior of the two materials is significantly different. While paper on initial loading behaves as a quasi-elastic material, the siliceous material behaves inelastically. However, after initial inelastic deformation, the siliceous material behaves elastically on all subsequent cycles until the maximum stress to which the material was prestressed is exceeded and beyond which the material behaves inelastically again. Thus, below the prestress limit that should be about 20% greater than the maximum stress to which the material will be subjected in use, the material behaves as an elastic material whose properties vary only slightly with depth.
The plane-wave impedance of longitudinal waves is Z=.rho.c, where .rho. is the density of the material and c is the velocity of sound in the material. A comparative measurement was made of the variation of density and velocity in the paper and the siliceous material. The variation of impedance with pressure for these materials during the initial stress cycle is as follows, qualitatively. Although both materials have impedance which increases monotonically with pressure, the MIN-K starts at a lower value and increases at a lower rate. However, since the deformation of the siliceous material is inelastic during this first pressure cycle, the material's behavior is much different in subsequent cycles in which the stress is limted to smaller values than the prestress. In this case, at zero stress, the paper has the smaller initial impedance, and the initial impedance of the siliceous material is determined by the prestress. Considering the variation of the elastic behavior of the siliceous material for different prestress values, both the zero stress impedance and the slope are different for each prestress. However, for stresses greater than 2,000 psi, the impedance of the siliceous materials is always less than that of paper even when the siliceous material has been prestressed to 12,000 psi.
Since the velocity and impedance curves for siliceous material are nearly straight lines, they are completely specified by their slopes and zero stress values, which are functions of the prestress. If curves of these slopes and zero stress values are plotted, the performance of any prestressed formulation of the siliceous material may be inferred.
Since the stress in the pressure-release material is greater than the external hydrostatic pressure (by a factor given by the ratio of the area of that part of the element head exposed to the water to the are of that part of the element head backed by the siliceous material), the prestress must be much greater than the pressure of maximum operational depth. Thus, the prestress in the siliceous material at the maximum depth of 4,500 feet is 6,000 psi. In order to stabilize the siliceous material up to the 6,000 psi maximum operating stress, it is prestressed an additional 20% to 7,200 psi.